[FLINK-4560] [build] Enforcer Java version >= 1.7 via Maven enforcer …

…plugin

This closes apache#2458

pom.xml

@@ -96,6 +96,7 @@ under the License.
  <slf4j.version>1.7.7</slf4j.version>
  <guava.version>18.0</guava.version>
  <akka.version>2.3.7</akka.version>
+ <java.version>1.7</java.version>
  <scala.macros.version>2.0.1</scala.macros.version>
  <!-- Default scala versions, may be overwritten by build profiles -->
  <scala.version>2.10.4</scala.version>
@@ -929,8 +930,8 @@ under the License.
  <artifactId>maven-compiler-plugin</artifactId>
  <version>3.1</version><!--$NO-MVN-MAN-VER$-->
  <configuration>
- <source>1.7</source>
- <target>1.7</target>
+ <source>${java.version}</source>
+ <target>${java.version}</target>
  <!-- The output of Xlint is not shown by default, but we activate it for the QA bot
  to be able to get more warnings -->
  <compilerArgument>-Xlint:all</compilerArgument>
@@ -999,7 +1000,7 @@ under the License.
  <plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-enforcer-plugin</artifactId>
- <version>1.3.1</version><!--$NO-MVN-MAN-VER$-->
+ <version>1.4.1</version><!--$NO-MVN-MAN-VER$-->
  <executions>
  <execution>
  <id>enforce-maven</id>
@@ -1012,6 +1013,9 @@ under the License.
  <!-- enforce at least mvn version 3.0.3 -->
  <version>[3.0.3,)</version>
  </requireMavenVersion>
+ <requireJavaVersion>
+ <version>${java.version}</version>
+ </requireJavaVersion>
  </rules>
  </configuration>
  </execution>